The yeast translation release factors Mrf1p and Sup45p (eRF1) are methylated, respectively, by the methyltransferases Mtq1p and Mtq2p.

The translation release factors (RFs) RF1 and RF2 of Escherichia coli are methylated at the N5-glutamine of the GGQ motif by PrmC methyltransferase. This motif is conserved in organisms from bacteria to higher eukaryotes. The Saccharomyces cerevisiae RFs, mitochondrial Mrf1p and cytoplasmic Sup45p (eRF1), have sequence similarities to the bacterial RFs, including the potential site of glutamine methylation in the GGQ motif. A computational analysis revealed two yeast proteins, Mtq1p and Mtq2p, that have strong sequence similarity to PrmC. Mass spectrometric analysis demonstrated that Mtq1p and Mtq2p methylate Mrf1p and Sup45p, respectively, in vivo. A tryptic peptide of Mrf1p, GGQHVNTTDSAVR, containing the GGQ motif was found to be approximately 50% methylated at the glutamine residue in the normal strain but completely unmodified in the peptide from mtq1-Delta. Moreover, Mtq1p methyltransferase activity was observed in an in vitro assay. In similar experiments, it was determined that Mtq2p methylates Sup45p. The Sup45p methylation by Mtq2p was recently confirmed independently (Heurgue-Hamard, V., Champ, S., Mora, L., Merkulova-Rainon, T., Kisselev, L. L., and Buckingham, R. H. (2005) J. Biol. Chem. 280, 2439-2445). Analysis of the deletion mutants showed that although mtq1-Delta had only moderate growth defects on nonfermentable carbon sources, the mtq2-Delta had multiple phenotypes, including cold sensitivity and sensitivity to translation fidelity antibiotics paromomycin and geneticin, to high salt and calcium concentrations, to polymyxin B, and to caffeine. Also, the mitochondrial mit(-) mutation, cox2-V25, containing a premature stop mutation, was suppressed by mtq1-Delta. Most interestingly, the mtq2-Delta was significantly more resistant to the anti-microtubule drugs thiabendazole and benomyl, suggesting that Mtq2p may also methylate certain microtubule-related proteins.

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